The referenced publication, "Journal of the SMPTE", Vol. 66, pp. 184-188, describes several problems which arise when magnetic heads located on the head wheel of a video drum are to be swtiched, and solutions in connection therewith. During the course of development since this publication in 1957, mechanical signal transfer with slip rings has been replaced by inductive systems, since slip rings and slip ring segments which inherently permit switching, and the consequent slip ring brushes introduce noise signals. Additional difficulties arise in connection with rapid commutation, imperfect or variable contact resistance between brushes and rotating slip rings and the like. Signal transfer, thus, was carried out in non-contactless manner, with electronic circuitry.
One widely used system utilized transverse track recording, with four magnetic transducer heads located on the circumference of a head wheel, in which two transducer heads, each, were interconnected in pairs. The transducer heads usually, were located at diametrically opposite locations on the head wheel, and only one of the transducer heads was in contact with the magnetic tape at any time. A disk, rotating in synchronism with the head wheel, had bright and dark zones or segments located thereon, the number corresponding to the magnetic heads on the head wheel. The differentially optically responsive zones or segments cooperated with a photo cell which generated a square-wave voltage. The squarewave voltage was used to control gate circuitry which, in turn, effected switching the respective magnetic heads at those instants of time during which one television line was followed by another, that is, during the horizontal retrace and blanking interval. It was difficult to suppress noise signals since those magnetic heads which were not in contact with the tape were also connected in circuit. Additionally, difficulties arose in respect to cross-talk due to capacitative and other feedback which occurred, spuriously, upon operation of the system.
It is, of course, possible to electrically separate oppositely located magnetic heads from a common reproduction channel or reproduction signal path. The reproduction quality is enhanced with respect to surious noise signals, cross-talk, and distortion. Yet, the electronic material and circuitry requirements to insure precise switch-over of the respective reproduction channels, each time after rotation of 90.degree. of the head wheel, was large. The respective methods and circuits all required precise construction of the segmental control disk, and alignment of the segments with the segmental control disk, or a control drum. This, in turn, substantially increases the price of an apparatus which is so constructed.
It has also been proposed to substitute electromagnetic scanning rather than electro-optical scanning of the position of the head wheel or, rather, the phase angle of the head wheel with respect to a reference, by associating permanent magnets with the rotating head wheel, and scanning the passage of the permanent magnets by a fixed pick-up tansducer element, such as a pick-up coil.